Roche limit

The smallest distance that a fluid satellite can orbit from the center of
a planet without being torn apart by tidal
forces. For a satellite of negligible mass, zero tensile strength, and
the same mean density as its primary, in a circular orbit around its primary,
this critical distance is 2.44 times the radius of the primary. (For the
Moon, whose density is lower than that of Earth, the Roche limit would be
2.9 Earth radii.) In practice, since moons tend to be solid, the tensile
force of the rock and ice of which they are composed helps prevent their
breakup. Even so, the shattering of satellites in orbits well inside the
Roche limit may explain the origin of some planetary ring systems. The limit
is named after the French mathematician Édouard Roche (1820-1883)
who first described the theory behind it.